Aging is associated with disturbances of both sleep and circadian rhythms. Nocturnal melatonin levels in the elderly are often substantially lower than in younger people. Studies have shown that high pharmacological doses of melatonin produce sleepiness and reduce sleep latency. The proposed studies will test the hypothesis that sleep disturbances of the elderly can be related to deficiencies in nocturnal melatonin secretion, and that restoration of rhythmic, high amplitude variations in circulating melatonin can improve sleep quality and quantity in the elderly without diminishing mood or performance the following morning. Elderly (60 -75 years) men and women participants (N=60) with and without symptomatic and polygraphically-confirmed insomnia will be tested under four conditions (placebo, 0.1, 0.2, 0.3 mg melatonin). Sleep quality and quantity will be polygraphically assessed along with measures of alertness, sleepiness, mood and performance. Parallel measures of cortisol and body temperature will be taken.